Polymer electrolytes have received significant attention due to their potential advantages over traditional liquid electrolytes for some electrochemical systems. Conventional Li-ion electrochemical cells, for example, often include liquid organic solvents for the electrolyte component, which are susceptible to safety issues arising from volatility and flammability, particularly in large format batteries. In contrast, Li-ion polymer or Li polymer electrochemical cells incorporate solid or gel polymer electrolytes without a liquid organic solvent component, thereby mitigating these well-recognized safety concerns. Incorporation of solid electrolytes in lithium battery systems is also a viable approach for providing mechanical properties useful for addressing other potential problems in lithium and lithium ion electrochemical cells, such as dendrite induced electrical shorting and thermal runaway.
Ion conductivity in polymer electrolyte systems is commonly achieved via doping a polymer host with one or more sources of lithium ions and counter ions, such as doping with lithium salts. Polymer systems compatible with efficient complexation of lithium ions, for example, allow for loading of more salt and greater availability of Li ions. Recently, ionic liquids have drawn attention as a potential alternative to Li salts as a means to introduce lithium ions to a useful extent into the electrochemical system. Ionic liquids are molten salts at room temperature, comprising predominately ions. An ion gel electrolyte, for example, may be formed by mixing an ionic liquid with a polymer, thereby providing potential benefits over conventional liquid electrolytes including reduced flammability, low vapor pressure, thermal stability, low toxicity and high ionic conductivity.
Block copolymers are an attractive material for polymer electrolytes due to their ability to self-assemble to form supramolecular structures characterized by nanoscale domains. This property of block copolymers is useful for achieving solid electrolytes having a combination of useful ionic conductivity and mechanical properties. Due to their potentially beneficial mechanical, chemical and electrical properties, substantial research has been directed towards use of block copolymers as solid electrolytes for lithium and lithium ion battery systems (See, e.g., Young et al., Block Copolymer Electrolytes for Rechargeable Lithium Ion Batteries, J. Polym. Sci. Part B: Polym. Phys., 2014, 52, 1-16). Amphophilic block copolymers are of particular interest, for example, because they allow for the selection of the composition of different polymer blocks to achieve an extent of self-assembly useful to achieve a mechanically robust system.
The development of block copolymers for solid electrolytes is currently impeded by technical challenges, most notably in achieving ionic conductivities approaching that of conventional organic solvent based liquid electrolytes. Block copolymers having a polyethylene oxide (PEO) block, for example, have been proposed and evaluated as potentially providing solid electrolytes with enhanced ionic conductivity. While methods to increase the conductivity of block copolymer electrolytes have been attempted using PEO-containing block copolymers, these approaches have proven less effective given that PEO exhibits a complex crystallization behavior in block copolymer systems.
It can be seen from the foregoing that there remains a need in the art for copolymer systems which provide enhanced physical strength as well as increased ionic conductivity, specifically for use as electrolytes and electrocatalyst platforms.